1. Field of the Invention
The present invention relates to a fiber grating device in which a refractive index modulation is formed in a predetermined region along the longitudinal direction of an optical fiber, and an optical fiber suitable for realizing this optical fiber grating device.
2. Related Background Art
A fiber grating device is a device in which a refractive index modulation is formed in a predetermined region along it the longitudinal direction of an optical fiber. This fiber grating device can selectively reflect light having a wavelength xcexB which is represented by the expression of xcexB=2Nxc2x7xcex9, where N is the effective refractive index in the refractive index modulation region of the optical fiber, and xcex9 is the period of refractive index modulation. As a consequence, the fiber grating device is used as an optical filter, a multi/demultiplexer, and the like in a wavelength division multiplexing optical communication system and the like.
Such a fiber grating device is made as follows. First, a silica optical fiber whose core region is doped with GeO2 is made. The core region doped with GeO2 has such a UV photosensitivity that its refractive index rises in proportion to the amount of UV irradiation. Therefore, when the optical fiber is irradiated with UV rays whose intensity is spatially modulated along the longitudinal direction of the optical fiber, a refractive index modulation corresponding to the intensity modulation of UV rays is formed in the core region. Thus, a fiber grating device is made.
The inventors studied the conventional technique mentioned above and, as a result, have found the following problems. Namely, in practice, such a fiber grating device not only reflects the light having the wavelength xcexB but also causes a loss in light having other wavelengths (shorter than the wavelength xcexB) to be transmitted there through. This loss is known as a cladding mode coupling loss. The cladding mode coupling loss is assumed to occur due to the fact that a core mode light component having a specific wavelength shorter than the wavelength xcexB couples with a cladding region mode light component since, while a part of energy of light propagating through the optical fiber exists in its cladding region, the refractive index modulation is formed only in the core region.
Since this phenomenon of cladding mode coupling loss causes a loss in the light having a wavelength intended to be transmitted through the fiber grating device, it has been desired that the cladding mode coupling loss be reduced. It has been known that the cladding mode coupling loss can be reduced when a refractive index modulation is formed not only in the core region but also in a part of the cladding region near the core region (see Japanese Patent Application Laid-Open No. HEI 11-237514, and a literature, A. Inoue, et al., xe2x80x9cOptimization of Fiber Bragg Grating for Dense WDM Transmission System,xe2x80x9d IEICE Trans. Electron., Vol. E81-C, No. 8, pp. 1209-1218 (1998), for example). The conventional technique mentioned above, however, may not fully reduce the cladding mode coupling loss. The cladding mode coupling loss is only lowered to about 0.2 dB in the above-mentioned literature.
In order to overcome the problems mentioned above, it is an object of the present invention to provide a fiber grating device whose cladding mode coupling loss is reduced, and an optical fiber suitable for realizing such a fiber grating device.
The optical fiber in accordance with the present invention comprises (1) a core region having a first refractive index with a UV photosensitivity; (2) an inner cladding region surrounding the core region and having a second refractive index lower than the first refractive index with a UV photosensitivity; and (3) an outer cladding region surrounding the inner cladding region and having a third refractive index lower than the first refractive index. The UV photosensitivity in the inner cladding region has a deviation of xc2x110% or less. The fiber grating device in accordance with the present invention comprises the optical fiber in accordance with the present invention having a refractive index modulation formed in a predetermined region along the longitudinal direction thereof.
Since not only the core region but also the inner cladding region surrounding the core region has a UV photosensitivity, whereas the UW photosensitivity in the inner cladding region has a deviation of xc2x110% or less, the cladding mode coupling loss of the fiber grating device can fully be reduced. Here, xe2x80x9cUV photosensitivity has a deviation of xc2x110% or lessxe2x80x9d means that the UV photosensitivity at each position in the region falls within the range of 90% to 110% with respect to the average value. Though there are cases where the UV photosensitivity drops in practice in the vicinity of the boundary between the core region and the inner cladding region, such a drop will be neglected if its diametric width is not greater than the wavelength in use since it is not problematic in terms of characteristics of the diffraction grating in this case.
Preferably, the difference between the average value of UV photosensitivity in the core region and the average value of UV photosensitivity in the inner cladding region is 10% or less. Also, it is preferred that the deviation in UV photosensitivity over both of the core region and inner cladding region be xc2x110% or less. It is further preferred that the deviation in UV photosensitivity over both of the core region and inner cladding region be xc2x15% or less. Each of these cases is suitable for fully reducing the cladding mode coupling loss of the fiber grating device.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.